This is a good primer on Lebanon and Syria's intervention / occupation of that country from the Council on Foreign Relations.

Here is an excerpt on the role of the PLO:

What caused the war? Tensions among Lebanon’s Christians, Sunni Muslims, Shiite Muslims, and the Druze Muslim sect. Those groups had long jockeyed for power and influence. Under the French occupation, which lasted from the end of World War I until independence in 1943, a “confessional” system evolved that reserved certain government posts for each religious group. Under this system, Christians had the upper hand in the national assembly—the Chamber of Deputies—that chooses the president. This arrangement bred resentment among Lebanese Muslims, especially as they grew to outnumber Christians.

In the early 1970s, the arrival of Yasir Arafat and thousands of his fellow Palestinian Liberation Organization (PLO) militants exacerbated Christian-Muslim strains and swelled the Muslim ranks with thousands of experienced gunmen. Lebanese Muslim groups supported the PLO fighters, recently expelled from Jordan, while Maronite Christian groups worried that PLO raids against Israel would invite retaliation and destabilize Lebanon. Throughout the 1970s, the PLO increasingly used Lebanon as a base from which to attack Israel. Israeli forces invaded in 1978 and 1982; after the second invasion, they remained and occupied a strip of southern Lebanon for nearly 20 years. Egypt, Iraq, and Libya supported Muslim factions in the civil war, while the United States and Israel backed Christian groups. During the long course of the conflict, Syria alternately supported the Christian amd Muslim sides.

The PLO, alas, has largely been a cancer in the region, attempting and sometimes succeeding in destabilizing Israel, Jordan, and Lebanon. In part, the PLO was stopped from seizing control of Jordan by Israel's assistance. It is sad that so much of the world cannot see what the effects of this organization has been.

Had the Palestinians accepted the original partition in 1967, or been willing to make peace at various points, the region would have been saved much suffering.